1. Field of the Invention
This invention relates to an improvement of a calibrator used for a non-destructive optical measuring apparatus which can quantitatively determine specific components such as sugar contained in measurring objects such as peaches, citrous fruits (or oranges), grapes, tomatoes, muskmelons and watermelons, without destroying the measurring objects. More particularly, it relates to an improvement of a calibrator used for a non-destructive optical measuring apparatus of a light-transmission type.
2. Description of the Related Art
As non-destructive optical measuring apparatus of this type, non-destructive optical measuring apparatus making use of near infrared light are proposed in variety. In order to perform stable and highly precise measurement over a long term by the use of such measuring apparatus, calibrators are indispensable. This is because the long-term service of such non-destructive optical measuring apparatus tends to cause a lowering of measurement precision which is caused by deviations occurring in measuring systems (e.g., deviation of measuring light wavelength used and apparent variation in the amount of incident light and detecting light; the latter being caused by dust or the like having adhered to light-incident and light-emergent portions of the measuring apparatus). Accordingly, various calibrators and calibration methods have been studied and proposed for each of the measuring principle and constitution of non-destructive optical measuring apparatus.
Now, most of non-destructive optical measuring apparatus making use of near infrared light are constituted basically of a white-light source and a spectrometer provided inside the optical measuring apparatus. In such optical measuring apparatus, the light emitted from the white-light source is made incident on a measuring object and the light having reflected from its surface and the vicinity thereof is spectrally analyzed by means of the spectrometer to gain internal information of the measuring object.
In the calibration for non-destructive optical measuring apparatus of such a type, reference samples (references) made of inorganic materials free of changes with time have commonly been used, as exemplified by a glass diffuser panel and a fluorine resin piece. Such reference samples, however, have optical characteristics and temperature characteristics which are different from those of measuring objects in almost all cases, and hence it has been difficult to make highly precise calibration.
Meanwhile, a calibration method is also known in which a sample of the same kind as a measuring object is destructively inspected and the results obtained are compared with the results of non-destructive measurement of the measuring object. However, in an instance where fresh food such as vegetables and fruits are measuring objects, there are problems such that the time during which the sample is available is limited because, e.g., no sample is available in the market even if it is attempted to make calibration for a non-destructive optical measuring apparatus before fruits are actually selected, and not only the destructive measurement, which is manually made, takes a time but also a fairly large number of samples must be put to destructive measurement in order to level scatterings among samples.
Under such technical backgrounds, as disclosed in Japanese Patent Application Laid-open No. 9-15142 a means is proposed as a method of making spectral analysis by using a calibrator having substantially the same optical characteristics as a measuring object and almost not undergoing any changes with time in itself. More specifically, the calibrator (a false fruit) disclosed in this Japanese Patent Application Laid-open No. 9-15142 comprises a calibrator main body of a double-tube structure, which is so constructed that the space between double tubes in this calibrator main body is filled with an aqueous solution containing target components present in the measuring object and also an inner tube of the double tube is made to have a stated light reflectance, or that a suitable dispersoid is added in the filling aqueous solution. At the time of calibration for the non-destructive optical measuring apparatus, in the same way as the measurement on measuring objects, light is made incident on the calibrator from its surface and the light reflecting from the calibrator surface, the filling and the inner tube surface is spectrally analyzed so as to be utilized for the calibration.
Now, the above calibrator (a false fruit) has a structure which accords with the principle of reflection type measurement, and can be effective when used for non-destructive optical measuring apparatus of a reflection type. When, however, used for non-destructive optical measuring apparatus of a transmission type, it has a problem to be solved, as stated later.
More specifically, the non-destructive optical measuring apparatus are known to include reflection type non-destructive optical measuring apparatus in which the cast-back light, having reflected from the surface and the vicinity thereof, is spectrally analyzed to gain internal information of the measuring object as described previously, and transmission type non-destructive optical measuring apparatus in which light is made incident on a measuring object at its light-incident area and the light having entered, and having been transmitted through, the interior of the measuring object is detected at its light-emergent area set at a position different from the light-incident area (i.e., the reflected, cast-back light is not detected and only the transmitted light is detected) to gain the internal information of the measuring object by measuring its light absorption (e.g., absorbance or absorptivity coefficient).
Because of the above difference in type, there is a difference between the reflection type and transmission type non-destructive optical measuring apparatus as stated below. That is, the reflection type non-destructive optical measuring apparatus has had a problem that the light reflecting from the measuring object at its deep part is smaller in amount than the light reflecting from its surface or the vicinity thereof and hence the information of the measuring object at its deep part where the amount of light is relatively small can not successfully be evaluated. Stated specifically, in the case when measuring objects are vegetables and fruits such as muskmelons and watermelons, having thick rind, the information gained in the reflection type is mainly held by that on the rind and is thinly held by that on the sarcocarp. In the case of thin-rind vegetables and fruit, too, it has been difficult to well cope with the matter when it is intended to gain sufficient information on the deep part of the measuring object in respect of its inner-part spoiling, ripeness and so forth.
In contrast thereto, the transmission type non-destructive optical measuring apparatus is of the type that, as described above, the light (transmitted light) having been transmitted through the interior of the measuring object is detected at its position (light-emergent area) different from the light-incident area and the light absorption is measured to gain the internal information of the measuring object, and hence has an advantage that it can be free from the above problem even when the measuring objects are vegetables and fruits having thick rind or when the matter to be measured is the inner-part spoiling and ripeness of measuring objects having thin rind.
In the transmission type non-destructive optical measuring apparatus, however, it is important on account of analysis to know the physical distance at which the transmitted light has passed through the interior of the measuring object (hereinafter "effective light path length"; as distinguished from "light path length", the term "optical path length" used in general definition in physics is meant to be a value obtained when a physical distance at which the light has passed through a medium is multiplied by a refractive index of the medium. In the present specification, the "light path length" is meant to be a physical distance at which the light has traveled through the interior of a measuring object, not multiplied by its refractive index. In the present specification, the term "optical path length" is also used to mean the optical path length as used in the general definition). Accordingly, the light path length of a calibrator used for the transmission type non-destructive optical measuring apparatus has also had to be adjusted to the effective light path length of the measuring object.
More specifically, in the transmission type non-destructive optical measuring apparatus, as shown in FIG. 14, light with a wavelength .lambda. is made incident on a measuring object M such as a muskmelon, and the light having passed through the interior of the measuring object M is detected with a detector S, where specific components such as sugar present in the measuring object are quantitatively determined from, e.g., absorptivity coefficient .beta. (.lambda.) which is found according to the following expression (1): EQU P.sub.out (.lambda.)=P.sub.in (.lambda.)exp[-.beta.(.lambda.)L](1)
In the expression (1), P.sub.in (.lambda.) represents the amount of incident light made incident on the measuring object M, and P.sub.out (.lambda.) represents the amount of detecting light, detected with the detector S.
Since, however, the sarcocarp of vegetables and fruits such as muskmelons has light-diffusing properties, as shown in FIG. 14 the light with a wavelength .lambda., made incident on the measuring object M, does not passes at the shortest distance connecting the light-incident area and light-emergent area on the measuring object M, i.e., at a geometric light path length denoted by L, to go straight toward the detector S, but makes its way to the detector S at last while being scattered at various places inside the measuring object M. Namely, the light having entered the measuring object M travels at a larger light path length (effective light path length L') than the shortest geometric light path length (geometric distance) L. Hence, it follows that the light with a wavelength .lambda. is absorbed in excess by specific components such as sugar in the measuring object M, correspondingly to the longer distance at which the light with a wavelength .lambda. has traveled. More specifically, the absorptivity coefficient .beta. (.lambda.) found by using the geometric light path length (geometric distance L) in the expression (1) is not a true absorptivity coefficient but an apparent absorptivity coefficient, so that its value is larger than the value of a true absorptivity coefficient to tend to be a measured value different from the concentration of a specific component in the measuring object M. For this reason, in the transmission type non-destructive optical measuring apparatus, it is important on account of analysis to know the effective light path length at which the transmitted light has passed through the interior of the measuring object.
Thus, since in the transmission type non-destructive optical measuring apparatus it is important on account of analysis to know the physical distance (effective light path length L') at which the transmitted light has passed through the interior of the measuring object, the light path length of the calibrator used for the transmission type non-destructive optical measuring apparatus has had to be adjusted to the effective light path length of the measuring object. That is, since the deviations occurring in a measuring system are detected as deviations of absorbance and absorptivity coefficient, the calibrator and the measuring object must be made to have equal light path length in order to make equal the amount of deviations of absorbance and absorptivity coefficient which occur in the same measuring system. Namely, this is because the effective light path lengths of the calibrator and measuring object must be adjusted to each other before the calibrator can correct any deviations having occurred in the results of measurement of specific components present in the measuring object.
In the calibrator for the reflection type, disclosed in the above Japanese Patent Application Laid-open No. 9-15142, the effective light path length can not be set at a proper value because of its structure and hence, if used as a calibrator for the transmission type as it is, it has a problem of a difficulty in its practical application.